1. Field of the Invention
This invention relates to icing sensing systems for surfaces of vehicle structures, for instance aircraft structures and, more particularly, to an apparatus and a method for the detection of icing onset and ice thickness using ultrasonic echo ranging techniques.
2. Prior Art and Other Considerations
In general, icing upon surfaces of vehicles causes a variety of operational problems and may result in catastropic and life-threatening situations. In particular, for example, dangers of icing on aircraft surfaces are well known, and various measures are taken to reduce, avoid, and removing icing. In order to provide warning and quantitative indication of the degree and severity of ice accretion to facilitate initiation of such measures, whether under control of an operator or under automatic control, reliable and accurate detection of icing is an essential prerequisite.
Various ice detection devices are known in the art. For example, monitoring probes extending above the surfaces subject to icing provide some measure of icing, albeit in rather inaccurate and sometimes unreliable manners. Not only are protrusions above surfaces often undesirable and subject to damage, but such probes encounter icing conditions that are usually significantly different from the conditions prevailing upon actual vehicle surfaces. Another known is based upon measurement of surface capacitance changes that occur as a function of changes in the thickness of ice (the ice layer acting as a dielectric). However, the dielectric properties of ice are subject to significant variations depending on the type of ice, as for instance given by slush, glaze, rime etc., as well as by inclusions of dirt, dust, salts, and other materials. Consequently, capacitance measurement techniques have been relatively inaccurate. Optical methods have also been used, but are known to be hampered by optical-path obstructions due to substantially unavoidable dirt accretion.
A device for measurement of ice thickness employing ultrasonic, pulse-echo techniques is disclosed in U.S. Pat. No. 4,628,736 issued to Kirby et al. Kirby et al utilizes ultrasonic compression waves emitted into the ice by a transducer disposed at the accretion surface.. Reflections of the waves from the ice/air interface are received and detected by the transducer, and the time delay between emission and reception of the signals provides a measure related to the thickness of the ice.
It is well known in the art that compressional sound waves do not distinguish adequately between transmission into ice and water. Compression waves propagating into a water layer and reflecting off the water/air interface are practically indistinguishable from those propagating into an ice layer and reflecting from the ice/air interface. Thus, whereas the device taught by Kirby et al recognizes accretion on a surface layer, it is unable adequately to distinguish whether the layer is water or ice. Moreover, since the transducer device taught by Kirby et al is disposed at the accretion surface, thin accretion layers are not practically detectable because resulting reflected waves are received by the transducer while it is still saturated and ringing due to the initial transmission signal, whereby reliable detection of icing onset and of trace amounts of ice is precluded.
The apparatus and method of the present invention are objects to overcome the difficulties of the prior art and provide for the sensitive detection of the very onset of icing as well as the measurement of ice thickness and the distinction between water and ice.